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<H1><A HREF="../vri.html">V.R.I.  -  Virtual Radio Interferometer</A></H1>

The <EM>Virtual Radio Interferometer</EM> (VRI) is an experimental
Java applet written by Nuria McKay, Derek McKay and Mark Wieringa,
from the 
<A HREF="http://www.jb.man.ac.uk"> 
  Nuffield Radio Astronomy Laboratories, Jodrell Bank
</A> and the 
<A HREF="http://wwwnar.atnf.csiro.au"> 
  Australia Telescope Compact Array
</A>.
<P>
It simulates various radio interferometers, and allows users to
conduct a number of "what-if" experiments. It extends the prototype
version (
<A HREF="http://wwwnar.atnf.csiro.au/astronomy/uvTest.html">
  uvTest
</A>) by allowing accumulation of arrays, improved control and fourier 
transforms.

<H2>Basic UV operation</H2>

The VRI appletis divided into four panels, (clockwise from top right)
the observatory map, the UV coverage plot, the UV plane and the image
plane. Each of these panels can be scaled (using the <B>Zoom</B> buttons or
the PGUP or PGDN keys) and scrolled (using the arrow keys). The
observatory map and UV coverage panels will show the scale by means of
a scale bar. Configurations can be selected from the configuration
menu.
<P>
The <B>Plot</B> button forces the current array layout to be plotted in the
UV coverage panel. The UV coverage is also affected by the <B>Hour Angle</B>,
<B>Declination</B> and <B>Frequency</B> selection at the bottom of the applet.
<P>
UV coverage from different arrays can be accumulated, byt pressing the
<B>Add</B> button after each array has been selected. Old arrays can be
plotted in blue or red and can be hidden, by selecting the appropriate
option out of the <B>Accumulate</B> menu. The <B>Clear</B> 
button will remove any
stored arrays.

<H2>Image and UV plane control</H2>

VRI allows the user to also experiment with test images. The
<B>Source</B> menu will allow the selection of different test
sources. By selecting <B>file:</B>, alternative test sources can be
entered. These should be .GIF files, no greater that 256x256 pixels.
<P>
Clicking on the <B>FFT</B> will casue the fourier transform of the
current image to be displayed in the UV plane panel. Conversely,
clicking on the <B>FFT-1</B> button will cause the inverse fourier
transform of the UV plane to be displayed in the image panel.
<P>
The UV plane, comprising complex numbers, can be displayed in various
forms. Choose the appropriate <B>Display</B> from the menu and wait a
few seconds for the UV plane to be replotted.

<H2>Applying UV coverage</H2>

By pressing the <B>Apply</B> button, the current UV plane is masked by
the current and accumulated UV coverage. By inverse transforming the
masked UV plane, a simulation of the image, as if observed by such a
synthesised aperture, can be obtained. As the scale of the images is
arbitrary, the masking is down at the scaled level, allowing vastly
difference arrays to be simulated (e.g. VSOP vs. ATCA). To compare
arrays, make sure the UV plane and coverage displays are set at the
scale for each application.

<H2>Other observatories</H2>

The <B>Obs:</B> menu and fields allow the user to control which
observatory is used. Arbitrary observatories can be created, by
adjusting the values in the various fields and pressing return for
each alteration. It is also possible to drag the antennas about the
observatory map, using the mouse, to simulate any configuration. New
antennas are always "piled up" at the centre of the array, and will
need to be moved. Don't use too many antennas, it will take forever to
plot!
<P>
The authors can add additional observatories if there is enough
demand. The required information we need is:
<UL>
<LI> Reference point (latitude and longitude of observatory)
<LI> Number of antennas
<LI> Antenna diameter
<LI> Antenna elevation limit
<LI> Station positions (with respect to the reference point)
<LI> Stations between which, there is railway track
<LI> Configurations (name plus station numbers)
</UL>

<H2>Notes</H2>

The FFT, FFT-1, Plot and Display options all deal with a lot of data
(especially for an interpretted language). 
<P>
While the authors hope that this applet will be useful, it is not a
definative scientific tool. For more detailed analysis of UV coverage,
etc. use 
<A HREF="http://wwwatnf.atnf.csiro.au/Software/Analysis/miriad/">Miriad</A>,
in particular, the
<A HREF="http://wwwatnf.atnf.csiro.au/Software/Analysis/miriad/uvgen.html">
UVGEN</A> task.
<P>
There is a bug in some versions of NetScape, which prevents
recognition of keyboard keys like PGUP, PGDN, etc.
<P>
Sometimes, because of slow links, the test source images don't fully
load. Just select them again. Also, the download time is fairly long,
owing to the large amounts of code - an added incentive to visit the ATCA!

<H2>Comments?</H2>

We would greatly appreciate any comments you may have on this
documentation, or the VRI java applet.
Please e-mail them to Nuria McKay
(<A HREF="mailto:nm@jb.man.ac.uk"><CODE>nm@jb.man.ac.uk</CODE></A>).

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<ADDRESS>
Original: dmckay@atnf.csiro.au (4-FEB-1997)<BR>
Modified: nm@jb.man.ac.uk (10-Sep-1997)
</ADDRESS>
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